testing/infra/builder: call make with empty env

testing/infra/builder: call make with empty env

Use an empty environment when calling make, but import PATH so the
buildroot tree under test can find binaries from the host machine.

Since environment variables are now ignored, move the handling of
BR2_DL_DIR to the defconfig to keep the current precedence of -d:

    BR2_DL_DIR  | -d DIR   | test downloads   | BR downloads
    ------------+----------+------------------+--------------
    unset       | unset    | [error]          | [error]
    unset       | set      | in $(DIR)        | in $(DIR)
    set         | unset    | in $(BR2_DL_DIR) | in $(BR2_DL_DIR)
    set         | set      | in $(DIR)        | in $(DIR)

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>